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Full text of "On the Direction of the Radicle and Germen during the Vegetation of Seeds"

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W, On the Direction of the Radicle and Gernien during the 
Vegetation of Seeds. By Thomas Andrew Knight, Esq. 
F. R, 8, In a Letter to the Right Hon. Sir Joseph Banks, 



Read Janua'ry 9, 1806. 



MY DEAR SIR, 

It can scarcely have escaped the notice of the most inatten- 
tive observer of vegetation, that in whatever position a seed is 
placed to germinate, its radicle invariably makes an effort to 
descend towards the centre of the earth, whilst the elongated 
germen takes a precisely opposite direction ; and it has been 
proved by Du Hamel* that if a seed, during its germination, 
be frequently inverted, the points both of the radicle and 
germen will return to the first direction. Some naturalists 
have supposed these opposite effects to be produced by gra- 
vitation ; and it is not difficult to conceive that the same agent* 
by operating on bodies so differently organized as the radicle 
and germen of plants are, may occasion the one to descend 
and the other to ascend. 

The hypothesis of these naturalists does not, however, appear 
to have been much strengthed by any facts they were able 
to adduce in support of it, nor much weakened by the argu- 
ments of their opponents ; and therefore, as the phenomena 

* Pbysigue deg Arhres* 



joo Mr. Knight on the Direction of 

observable during the conversion of a seed into a plant are 
amongst the most interesting that occur in vegetation, I 
commenced the experiments, an account of which I have now 
the honour to request you to lay before the Royal Society, 

I conceived that if gravitation were the cause of the descent 
of the radicle, and of the ascent of the germen, it must act 
either by its immediate influence on the vegetable fibres and 
vessels during their formation, or on the motion and conse- 
quent distribution of the true sap afforded by the cotyledons ; 
and as gravitation could produce these effects only whilst the 
seed remained at rest, and in the same position relative to 
the attraction of the earth, I imagined that its operation would 
become suspended by constant and rapid change of the posi- 
tion of the germinating seed, and that it might be counter- 
acted by the agency of centrifugal force. 

Having a strong rill of water passing through my garden., 
I constructed a small wheel similar to those used for grinding 
corn, adapting another wheel of a different construction, and 
formed of very slender pieces of wood, to the same axis 
Round the circumference of the latter, which was eleven 
inches in diameter, numerous seeds of the garden bean, which 
had been soaked in water to produce their greatest degree of 
expansion, were bound, at short distances from each other. 
The radicles of these seeds were made to point in every 
direction, some towards the centre of the wheel, and others 
in the opposite direction; others as tangents to its curve, 
some pointing backwards, and others forwards, relative to its 
motion; and others pointing in opposite directions in lines 
parallel with the axis of the wheels. The whole was inclosed 
in a box^ and secured by a lock, and a wire grate was placed 



the Radicle and Gertnen of Seeds, 101 

to prevent the ingress of any body capable of impeding the 
motion of the wheels. 

The water being then admitted, the wheels performed 
something more than igo revolutions in a minute; and the 
position of the seeds relative to the earth was of course as 
often perfectly inverted, within the same period of time ; by 
which I conceive that the influence of gravitation must have* 
been wholly suspended. 

In a few days the seeds began to germinate, and as the 
truth of some of the opinions I had communicated to you, and 
of many others which I had long entertained, depended on 
the result of the experiment, I watched its progress with 
some anxiety, though not with much apprehension; and I 
had soon the pleasure to see that the radicles, in whatever 
direction they were protruded from the position of the seed, 
turned their points outwards from the circumference of the 
wheel, and in their subsequent growth receded nearly at right 
angles from its axis. The germens, on the contrary, took 
the opposite direction, and in a few days their points all met 
in the centre of the wheel. Three of these plants were suf- 
fered to remain on the wheel, and were secured to its spokes 
to prevent their being shaken off by its motion. The stems 
of these plants soon extended beyond the centre of the wheel : 
but the same cause, which first occasioned them to approach 
its axis, still operating, their points returned and met again at 
its centre. 

The motion of the wheel being in this experiment vertical, 
the radicle and germen of every seed occupied, during a 
minute portion of time in each revolution, precisely the same 
position they would have assumed had the seeds vegetated at 



lo© Mr. Knight on the Direction of 

rest ; and as gravitation and centrifugal force also acted lit 
lines parallel with the vertical motion and surface of the 
wheel, I conceived that some slight objections might be urged 
against the conclusions I felt inclined to draw. I therefore 
added to the machinery I have described another wheels 
which moved horizontally over the vertical wheels ; and to 
this, by means of multiplying wheels of different powers, I 
was enabled to give many different degrees of velocity. 
Round the circumference of the horizontal wheel, whose dia- 
meter was also eleven inches, seeds of the bean were bound 
as in the experiment, which I have already described, and it 
was then made to perform 250 revolutions in a minute. By 
the rapid motion of the water-wheel much water was thrown 
upwards on the horizontal wheel, part of which supplied the 
seeds upon it with moisture, and the remainder was dis- 
persed, in a light and constant shower, over the seeds in the 
vertical wheel, and on others placed to vegetate at rest in 
different parts of the box. 

Every seed on the horizontal wheel, though moving with 
great rapidity, necessarily retained the same position relative 
to the attraction of the earth ; and therefore the operation of 
gravitation could not be suspended, though it might be coun- 
teracted, in a very considerable degree, by centrifugal force : 
and the difference, I had anticipated, between the effects of 
rapid vertical and horizontal motion soon became sufficiently 
obvious, The radicles pointed downwards about ten degrees 
below, and the germens as many degrees above, the horizontal 
line of the wheel's motion ; centrifugal force having made 
both to deviate 80 degrees from the perpendicular direction 
each would have taken, had it vegetated at rest, Gradually 



the Radicle and Germen of Seeds, j 03 

diminishing the rapidity of the motion of the horizontal 
wheel, the radicles descended more perpendicularly, and the 
germens grew more ijpright ; and when it did not perform 
more than 80 revolutions in a minute, the radicle pointed 
about 45 degrees below, and the germen as much above* 
the horizontal line, the one always receding from, and the 
other approaching to, the axis of the wheel. 

I would not, however, be understood to assert that the 
velocity of 250, or of 80 horizontal revolutions in a minute 
will always give accurately the degrees of depression and 
elevation of the radicle and germen which I have mentioned ; 
for the rapidity of the motion of my wheels was sometimes 
diminished by the collection of fibres of conferva against the 
wire grate ; which obstructed in some degree the passage of 
the water : and the machinery, having been the workmanship 
of myself and my gardener, can not be supposed to have 
moved with all the regularity it might have done, had it been 
made by a professional mechanic. But I conceive myself to 
have fully proved that the radicles of germinating seeds are 
made to descend, and their germens to ascend, by some ex- 
ternal cause, and not by any power inherent in vegetable 
life : and I see little reason to doubt that gravitation is the 
principal, if not the only agent employed, in this case, by 
nature. I shall therefore endeavour to point out the means 
by which I conceive the same agent may produce effects so 
diametrically opposite to each other, 
"The radicle of a germinating seed (as many naturalists 
.-have observed) is 'Increased in length only .by : new parts 
successively added to its apex .or 'point, and not at all by any 



io§ Mr. Knight on the Direction of 

general extension of parts already formed: and the new 
matter which is thus successively added unquestionably de- 
scends in a fluid state from the cotyledons.^ On this fluid, 
and on the vegetable fibres and vessels whilst soft and 
flexible, and whilst the matter which composes them is 
changing from a fluid to a solid state, gravitation, I conceive, 
would operate sufficiently to give an inclination downwards 
to the point of the radicle ; and as the radicle has been proved 
to be obedient to centrifugal force, it can scarcely be con- 
tended that its direction would remain uninfluenced by gra- 
vitation. 

I have stated that the radicle is increased in length only by 
parts successively added to its point: the germen, on the 
contrary, elongates by a general extension of its parts pre- 
viously organized ; and its vessels and fibres appear to extend 
themselves in proportion to the quantity of nutriment they 
receive. If the motion and consequent distribution of the true 
sap be influenced by gravitation, it follows, that when the 
germen at its first emission, or subsequently, deviates from a 
perpendicular direction, the sap must accumulate on its under 
side : and I have found in a great variety of experiments on 
the seeds of the horse chesnut, the bean, and other plants, 
when vegetating at rest, that the vessels and fibres on the 
under side of the germen invariably elongate much more 
rapidly than those on its upper side ; and thence it follows 
that the point of the germen must always turn upwards. 
And it has been proved that a similar increase of growth 
takes place on the external side of the germen when the sap 

* See Phil. Trans, of 1805. 



the Radicle and Germen of Seeds, 105 

is impelled there by centrifugal force, as it is attracted by. 
gravitation to its under side, when the seed germinates at 
rest. 

This increased elongation of the fibres and vessels of the 
under side is not confined to the germens, nor even to the 
annual shoots of trees, but occurs and produces the most ex- 
tensive effects in the subsequent growth of their trunks and 
branches. The immediate effect of gravitation is certainly to 
occasion the further depression of every branch, which extends 
horizontally from the trunk of the tree ; and, when a young 
tree inclines to either side, to increase that inclination : but it 
at the same time, attracts the sap to the under side, and thus 
occasions an increased longitudinal extension of the substance 
of the new wood on that side.* The depression of the lateral 
branch is thus prevented ; and it is even enabled to raise itself 
above its natural level, when the branches above it are 
removed ; and the young tree, by the same means, becomes 
more upright, in direct opposition to the immediate action of 
gravitation : nature, as usual, executing the most important 
operations by the most simple means. 

I could adduce many more facts in support of the preceding 
deductions, but those I have stated, I conceive to be suffi- 
ciently conclusive. It has however been objected by Du 
Hamel, (and the greatest deference is always due to his 
opinions,) that gravitation could have little influence on 
the direction of the germen, w r ere it in the first instance 
protruded, or were it subsequently inverted, and made to 

* This effect does not appear to be produced in what are called weeping trees; 
the cause of which I have endeavoured to point out in a former Memoir. Phil. 
Trans. 1804, 

MDCCCVI. P 



io6 Mr. Knight on the Direction of 

point perpendicularly downwards. To enable myself to 
answer this objection, I made many experiments on seeds of 
the horse chesnut, and of the bean, in the box I have already 
described ; and as the seeds there were suspended out of the 
earth, I could regularly watch the progress of every effort 
made by the radicle and germ en to change their positions. 
The extremity of the radicle of the bean, when made to point 
perpendicularly upwards, generally formed a considerable 
curvature within three or four hours, when the weather was 
warm. The germen was more sluggish ; but it rarely or 
never failed to change its direction in the course of twenty- 
four hours ; and all my efforts to make it grow downwards, 
by slightly changing its direction, were invariably abortive. 

Another, and apparently a more weighty, objection to the 
preceding hypothesis, (if applied to the subsequent growth 
and forms of trees,) arises from the facts that few of their 
branches rise perpendicularly upwards, and that their roots 
always spread horizontally ; but this objection I think may 
be readily answered. 

The luxuriant shoots of trees, which abound in sap, in 
whatever direction they are first protruded, almost uniformly 
turn upwards, and endeavour to acquire a perpendicular di- 
rection ; and to this their points will immediately return, if 
they are bent downwards during any period of their growth ; 
their curvature upwards being occasioned by an increased 
extension of the fibres and vessels of their under sides, as in 
the elongated germens of seeds. The more feeble and slender 
shoots of the same trees will, on the contrary, grow in almost 
every direction, probably because their fibres, being more 
and their vessels less amply supplied with sap, they are 



the Radicle and Germen of Seeds. 107 

ess affected by gravitation. Their points, however, generally 
shew an inclination to turn upwards ; but the operation of 
light, in this case, has been proved by Bonnet* to be very 
considerable. 

The radicle tapers rapidly, as it descends into the earth, 
and its lower part is much compressed by the greater solidity 
of the mould into which it penetrates. The true sap also con- 
tinues to descend from the cotyledons and leaves, and occa- 
sions a continued increase of the growth of the upper parts of 
the radicle, and this growth is subsequently augmented by 
the effects of motion, when the germen has risen above the 
ground. The true sap is therefore necessarily obstructed in 
its descent ; numerous lateral roots are generated, into which 
a portion of the descending sap enters. The substance of 
these roots, like that of the slender horizontal branches, is 
much less succulent than that of the radicle first emitted, and 
they are in consequence less obedient to gravitation: and 
therefore meeting less resistance from the superficial soil, 
than from that beneath it, they extend horizontally in every 
direction, growing with most rapidity, and producing the 
greatest number of ramifications, wherever they find most 
warmth, and a soil best adapted to nourish the tree. As these 
horizontal, or lateral, roots surround the base of the tree on 
every side, the true sap descending down its bark, enters 
almost exclusively into them, and the first perpendicular 
root, having executed its office of securing moisture to the 
plant, whilst young, is thus deprived of proper nutriment, 
and, ceasing almost wholly to grow, becomes of no import- 
ance to the tree. The tap root of the oak, about which so 

* Recbercbes sur I'Usage des Feuilles dans les Plantes* 

P 2 



io8 Mr. Knight on the Direction of Radicles, &c. 

much has been written, will possibly be adduced as an excep- 
tion ; but having attentively examined at least 20,000 trees 
of this species, many of which had grown in some of the 
deepest and most favourable soils of England, and never 
having found a single tree possessing a tap root, I must be 
allowed to doubt that one ever existed. 

As trees possess the power to turn the upper surfaces of 
their leaves, and the points of their shoots to the light, and 
their tendrils in any direction to attach themselves to conti- 
guous objects, it may be suspected that their lateral roots are 
by some means directed to any soil in their vicinity which is 
best calculated to nourish the plant, to which they belong ; 
and it is well known that much the greater part of the roots 
of an aquatic plant, which has grown in a dry soil, on the 
margin of a lake or river, have been found to point to the 
water ; whilst those of another species of tree which thrives 
best in a dry soil, have ' been ascertained to take an opposite 
direction : but the result of some experiments I have made is 
not favourable to this hypothesis, and I am rather inclined to 
believe that the roots disperse themselves in every direction, 
and only become most numerous where they find most em- 
ployment, and a soil best adapted to the species of plant. 
My experiments have not, however, been sufficiently varied, 
or numerous, to decide this question, which I propose 
make the subject of future investigation, 



I am, &c. 



rn 



r. a. Kb 



Elton, Nov, 22, 1805,